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Hua Haifeng, who was working undercover with the two other men in Jiangxi province, has been held by police and accused of illegally using a wiretapping device

A labour activist investigating working conditions at a supplier for the Ivanka Trump brand in central China has been detained, and two of his colleagues are missing, their non-governmental organisation said Wednesday.

Hua Haifeng, who was working undercover with the two other men in Jiangxi province, has been held by police and accused of illegally using a wiretapping device, China Labor Watch director Li Qiang told AFP.

Deng Guilian, Hua's wife, was notified of his arrest by Jiangxi authorities on Tuesday. Hua and the two other activists, Li Zhao and Su Heng, have been unreachable since Sunday.

"In 17 years (since our organisation was founded), we have done countless such investigations and have never had any problems," Li said, noting that he suspects the reprisal may be related to the brand they were investigating.

"The accusation from Jiangxi police has no factual basis."

Li said the activists were probing two factories owned by major footwear producer Huajian Group -- one in the city of Dongguang in southern Guangdong province, the other in Jiangxi's Ganzhou.

"We appeal to President Trump, Ivanka Trump herself, and to her related brand company to advocate and press for the release of our activists," Li said.

Aside from Ivanka Trump -- the daughter of the US President and also an advisor -- the factory also manufactures products for other overseas brands, including Coach, Nine West, Karl Lagerfeld, and Kendall + Kylie.